This invention relates to apparatus for centrifugal separation, and more particularly, to apparatus for separating fine (less than 600 microns) coal/mineral particles having a relatively low density and high carbon content from coal/mineral particles having a relatively high density and therefore high ash content. The invention also relates to any other fine size solid particulate material or ore in which a density fractionation is desired.
Coal is often processed before combustion to remove undesired particles having a relatively high ash and/or mineral content from particles having a relatively high carbon content. Combustion efficiency is improved by processing, or cleaning, the coal in this manner because the high carbon particles burn well, but the high ash/mineral particles do not. Moreover, the high ash/mineral particles introduce undesired by-products into the atmosphere if they are not removed after combustion. For these reasons, efforts are made to remove the high ash/mineral particles before combustion.
When attempting to clean coal, the composition of the coal is periodically measured or characterized by fractionating the coal particles according to density. The separation can be made through gravimetric processes because the specific gravity of the ash/mineral particles is higher (about 1.8-2.6) than the specific gravity of the carbon particles (about 1.3).
The composition of a fine size coal sample can be determined by placing the coal in a liquid having a specific gravity of perhaps 1.2 to 3.0, and placing the solution in a centrifuge Heavy organic liquids such as perchloroethylene, naphtha or toluene are suitable for this purpose, but those liquids produce hazardous fumes in atmospheric conditions and should be utilized with caution.
As the centrifuge rotates the solution, low density particles having a specific gravity which is less than the specific gravity of the liquid form a "float" layer on the top of the solution, and high density particles form a "sink" layer on the bottom of the solution. A "clear" layer which consists primarily of the liquid is formed between the float and sink layers. The composition of the sample is determined by separately removing and weighing the float and sink layers.
Coal samples such as those just described can be placed in hourglass flasks for centrifugal separation. Such flasks are generally cylindrical, but have a narrow diameter in the neck of the flask. A sample solution is poured into the flask to about the top of the flask, and the flask is placed upright in the centrifuge without a cover. In the centrifuge, the high carbon particles in the solution float to the top of the solution, and the high ash/mineral particles sink to the bottom. The flask is then removed from the centrifuge and carried to a vented work area.
At the work area, a rubber stop which has the diameter of the neck of the hourglass flask is pushed through the float layer and secured in the neck of the flask to seal the neck. The float layer is then poured out of the flask so that the high carbon particles can be filtered from the solution and weighed. The rubber stop is then removed so that the sink layer can be poured out of the flask and measured in the same manner.
One problem with hourglass flasks is that the rubber stop upsets the float layer as the stop enters the flask, reducing the accuracy and reliability of the results. In addition, this technique is tedious and time-consuming. Thus, there is a need for apparatus for separating high carbon coal particles from high ash/mineral coal particles in gravimetric processes which provides more accurate, reliable results, quickly and easily.
Another problem with hourglass flasks is that hazardous fumes are released when the flask is in the centrifuge, and when it is moved from the centrifuge to the vented work area. Thus, there is also a need for apparatus for separating high carbon coal particles from high ash/mineral coal particles in gravimetric processes which do not release hazardous fumes in unvented areas.
Accordingly, one object of this invention is to provide new and improved techniques and apparatus for centrifugal float/sink separation.
Another object is to provide new and improved gravimetric cells for the centrifugal separation of coal/mineral matter which provide accurate, reliable results.
Yet another object is to provide new and improved gravimetric cells for the centrifugal separation of coal which produce more efficient separations in a less tedious manner in less time.
Still another object is to provide new and improved gravimetric cells for the centrifugal separation of coal/mineral matter which do not release hazardous fumes in unvented areas.